5 Damage induced changes in SESAM properties and new design concepts
5.2 Design concept to overcome Q-switch induced damage on SESAMsdamage on SESAMs
5.2.2 Experiments on the temperature dependence of SESAM propertiesproperties
Since we do not have the opportunity to grow SESAMs, we could not test our above-mentioned design idea so far. Nevertheless, we tested the temperature dependence of commercially available SESAMs in terms of the static reflectivity, the nonlinear re-flectivity curve, and the absorber recovery time. The SESAMs were studied with the FTIR-spectrometer and characterized with the pump-probe setup.
For the experiments we have chosen two different SESAMs: SAM-T1, a SESAM with a high effective modulation depth (∆Reff >20 %) as it is commonly used in fiber lasers and SAM-T2, a SESAM with a moderate effective modulation depth (∆Reff <5 %) as
5.2 Design concept to overcome Q-switch induced damage on SESAMs
900 950 1000 1050 1100 1150 1200
0 20 40 60 80 100
Reflectivity (%)
Wavelength (nm) 22°C
30°C 40°C 50°C 63°C 75°C 85°C 100°C
Figure 5.4: Static reflectivity of heated SAM-T1, measured with the FTIR-spectrometer. The reflectivity values at the Yb:YAG laser wavelength of 1030 nm (marked with the black line) are used as linear reflectivity values in further calcula-tions and fit routines.
it is commonly used for mode-locking thin-disk lasers.
The SESAMs were glued with silver paste on a heatable sample holder and the sur-face temperature was monitored using a thermal camera (Infratec VarioCam). SESAM temperatures up to 100◦C were achieved. The temperature range was limited by the heater resistor we used.
Temperature dependent reflectivity of a SESAM with a high modulation depth SAM-T1 is a SESAM with an effective modulation depth at room temperature of about 20 % and a relatively fast relaxation time constant of less than 20 ps. The quantum well material is In0.5Ga0.5As and the SESAM design is optimized for mode-locking lasers with a laser wavelength of 1040 nm.
The static, linear reflectivity spectrum of the SESAM was measured with the FTIR-spectrometer and changed drastically when the SESAM was heated. As it can be seen in Fig. 5.4 there are two main effects: With increasing temperature the reflectivity of the stopband decreases and the complete spectrum exhibits a redshift of approxi-mately 10 nm. The redshift is contributed to a temperature dependent increase of the refractive indexes of AlAs and GaAs [Bla82, Tal95, Kim07] leading to a shift of the spectrum towards lower energies, respectively higher wavelengths.
Since the SESAM design is optimized for a laser wavelength of 1040 nm, all spectra exhibit the stopband in the region between 1000-1100 nm. While the reflectivity of the stopband of SESAMs with moderate modulation depths typically shows a flat shape the stopband of SAM-T1 is dominated by a strong quantum well absorption around 1020 nm at room temperature, respectively 1030 nm when heated, as it is typical for SESAMs with a resonant coating [Kel10]. This leads to a reflectivity at 1030 nm below 75 % as it is required for a modulation depth of at least 20 %. With increasing tem-perature the reflectivity value at 1030 nm decreases from approximately 70 % down to 46 %. Due to the redshift the reflectivity at 1030 nm might exhibit a minimum around 100◦C and even raise again for further heating.
The strong temperature dependence of the stopband region of SAM-T1 is explained in the following. The decrease of the static reflectivity corresponds to an increase of the quantum well absorption. This might be caused by an increased field enhancement within the SESAM due to a heat induced change in the resonant coating. Additionally, as mentioned in Section 5.2 and given in Eq. (5.1), the band gap energy of In1−xGaxAs is strongly temperature dependent. For In0.5Ga0.5As the band gap energy at room temperature is ca. 0.78 eV and decreases approximately linearly with a slope of ca.
−0.07eV
20K , as it is plotted in the left part of Fig. 5.5. Hence, the wavelength range of the stopband between 1000-1100 nm, corresponding to an energy range from 1.13-1.24 eV, is above the band gap energy in a region of high absorption.
The absorption spectrum also exhibits a temperature dependent behavior as it can be seen in the absorption spectra of In0.2Ga0.8As/GaAs quantum wells in the right part of Fig. 5.5. Two effects should be pointed out: For increasing temperatures the spec-trum shifts towards lower energies due to the shift of the band edge energy and the absorption peaks broaden due to a stronger LO phonon interaction. For a given photon energy (here 1030 nm = 1.2 eV) this results in an increasing absorption, respectively∧ decreasing reflectivity with increasing temperature.
Another effect that changes the absorption spectra of InGaAs quantum wells is the strain within the structure. The lattice constant of In0.5Ga0.5As is 5.85 ˚A [Lev99]
whereas the lattice constant of GaAs is 5.65 ˚A [Yu03]. Consequently, the absorber re-gion of SAM-T1 is strained which leads to an additional shift of the absorption spectra towards lower energies [Yu03].
For SAM-T1 with a modulation depth of 20 % and in general for SESAMs with high modulation depth several quantum well absorbers are necessary. Therefore, the tem-perature dependent changes in the absorption of quantum wells, as they are mentioned above for single quantum wells, even amplify for SAM-T1 leading to such strong tem-perature dependent linear reflectivity spectra as they are shown in Fig. 5.4. The static reflectivity values at a wavelength of 1030 nm were extracted from the FTIR-spectra and are used as linear reflectivity Rlin in further calculations and fitting routines.
In a second step the SESAM was measured with the pump-probe spectroscopy setup.
As it is described in Chapter 4.4.4, the nonlinear reflectivity curves based on pump-probe data are no absolute values, but relative to the linear reflectivity Rlin. The absolute nonlinear reflectivity curves in Fig. 5.6 are based on the linear reflectivity
5.2 Design concept to overcome Q-switch induced damage on SESAMs
0 100 200 300 400
0.75 0.8 0.85 0.9 0.95 1 1.05 1.1
Temperature (K) E gap (eV)
In0.5Ga
0.5As In0.3Ga
0.7As
Energy (eV) ptoa. utAbsorin (rbnis)
Figure 5.5: Left: Temperature dependent band gap energy of In0.5Ga0.5As and In0.3Ga0.7As according to Equation (5.1). Right: Absorption spectra of In0.2Ga0.8As/GaAs-QWs for different temperatures. The figure is taken from [She95a].
The redshift of the spectra for increasing temperatures due to the decrease of the band edge energy is obviously. For the reason of clarity the curves are vertically shifted.
extracted from the FTIR-spectra. All characteristic parameters based on the pump-probe measurements and the static reflectivity spectra of the heated SAM-T1 are listed in detail in Table 5.1.
As it can be seen in Fig. 5.7 (left) the fit parameterRlin/Rns shows the same tempera-ture dependence as the linear reflectivity measured with the FTIR-spectrometer: With increasing temperature Rlin/Rns decreases and flattens for temperatures near 100◦C, which confirms the accuracy of the pump-probe measurements, the fit routines and the consistency between the different experimental methods. The nonlinear losses Rns were calculated by comparing the fit parameterρ=Rlin/Rns with the static reflectivity Rlin, FTIR at a wavelength of 1030 nm: Rns = Rlin,FTIR/ρ = Rlin, FTIR/(Rlin/Rns). The nonsaturable reflectivity remains almost constant over the temperature range, which confirms our expectations. Nonsaturable losses arise i.a. from absorption of free carri-ers and defects, from scattering losses and two-photon absorption [Kel99]. Since these effects show no strong temperature dependence, a temperature dependence of Rns is not expected. This hypothesis is confirmed, as it can be seen in Fig. 5.7 (left).
In the right part of Fig. 5.7 the effective modulation depth and the linear reflectivity is plotted versus SESAM temperature. The increase of the effective modulation depth
100 101 102 103 40
50 60 70 80 90 100
Reflectivity (%)
Fluence (µJ/cm2)
22°C 30°C 40°C 50°C 63°C 75°C 85°C 100°C
Figure 5.6: Nonlinear reflectivity curve of heated SAM-T1, experimental results (dots) and fit curve (line). Absolute values based on PPS data and linear reflectivity values extracted from FTIR-spectra.
follows directly from the decrease of the linear reflectivity and the temperature stability of the nonsaturable reflectivity.
The time dependent reflectivity transients of SAM-T1 for different temperatures are plotted in Fig. 5.8. The transients are normalized to extract the recombination time constant assuming an exponential decay. The transients correspond to low fluence values of 17µJ/cm2 so that induced absorption, e.g. two-photon absorption and free-carrier absorption, could hardly influence the free-carrier dynamics and the shape of the transients, respectively. Independent of the SESAM temperature the recombination time constants are < 15 ps. It is notable that all transients, except the transient measured at room temperature and at 100◦C, show a relatively constant decay time of about 5-7 ps. These values are within the measurement’s accuracy of a few picoseconds, which is limited by the laser pulse duration of 1 ps. In further temperature dependent time resolved pump-probe measurements of SAM-T1 the decay time at room temper-ature was also in the range of about 6 ps. That allows for the conclusion that the relatively slow decay in the 22◦C transient can be contributed to a bad spot and may be disregarded. Hence, for moderate SESAM temperatures the relaxation time con-stants do not show an obvious temperature dependence.
As it can be read in Section 3.3 and 3.4 about carrier dynamics in SESAMs the recombi-nation time is mainly determined by the amount of defect states within the absorption
5.2 Design concept to overcome Q-switch induced damage on SESAMs
Table 5.1: Characterizing parameters for heated SAM-T1 extracted from pump-probe measurements and static FTIR-spectra. The time constants are extracted from the transients at a fluence of 17 µJ/cm2, which are shown in Fig. 5.8.
20 40 60 80 100
Figure 5.7: Selected characterizing parameters for heated SAM-T1, temperature de-pendent. Left: Comparison of the fit parameter Rlin/Rns extracted from pump-probe measurements with the static, linear reflectivity values Rlin,FTIR measured with the FTIR-spectrometer. Based on these two parameters the nonsaturable reflectivity Rns
was calculated. Right: Linear reflectivity Rlin and effective modulation depth ∆Reff versus SESAM temperature.
region. The recombination time constant can therefore be increased by annealing these defects [Sch12a]. The transients in Fig. 5.8 lead to the assumption that temperatures below 100◦C do not influence the defect states and the carrier dynamics respectively.
The slow decay of 9 ps of the transient at 100◦C compared to the decay time of 5 ps of the transients at lower temperatures may already be due to annealing effects.
Temperature dependent reflectivity of a SESAM with a low modulation depth We also characterized a SESAM with a low modulation depth, labeled SAM-T2, at different temperatures. The effective modulation depth of SAM-T2 at room temper-ature is 3 % and the recombination time constant is faster than 30 ps. The quantum well material of SAM-T2 is In0.3Ga0.7As0.985N0.015. The SESAM design is optimized for
0 10 20 30 40 50 60 0
0.2 0.4 0.6 0.8 1
1/e
∆ R/R lin (norm.)
Time delay (ps)
22°C 30°C 40°C 50°C 63°C 75°C 85°C 100°C
0 5 10 15 20
0.3 0.35 0.4 0.45
Figure 5.8: Normalized reflectivity transients of SAM-T1 at a fluence of 17 µJ/cm2 for different SESAM temperatures. The inset shows the zoom into the transients, where the time constant of the exponential decay can clearly be extracted. The horizontal black line marks a reflectivity decrease of 1/e.
mode-locking thin-disk lasers at a wavelength of 1030 nm.
The static reflectivity spectra of SAM-T2, which are shown in Fig. 5.9, do not change as drastically as the spectra of SAM-T1. The only obvious effect of the heating is the redshift of the spectra by 10 nm, analogous to the redshift of the spectra of SAM-T1.
This redshift leads to a decrease of approximately 4 % of the reflectivity at a wavelength of 1030 nm, since this is close to the stopband edge.
In contrast to SAM-T1, all spectra of SAM-T2 exhibit flat stopbands between 1010 nm and 1110 nm. Photoluminescence measurements confirmed the excitonic absorption of SAM-T2 far outside the stopband in the wavelength range of 1200 nm. Hence, in the FTIR-spectra of SAM-T2 no strong absorption dip is visible. In the following some reasons for the unchanged flat stopband of SAM-T2 are given. As already mentioned the quantum well material of SAM-T2 is In0.3Ga0.7As0.985N0.015. The band gap en-ergy at room temperature of pure In0.3Ga0.7As according to Equation (5.1) is 1.01 eV.
The band gap energy for In0.5Ga0.5As and In0.3Ga0.7As as a function of temperature is plotted in the left panel of Fig. 5.5. Due to nitrogen incorporation the band gap energy decreases and is approximately 0.9 eV for In0.3Ga0.7As0.985N0.015 at room tem-perature [Zha05]. Hence, the energy of 1.2 eV, corresponding to the laser wavelength of 1030 nm, is above the band edge within the absorption range, but not as far as for SAM-T1 whose In0.5Ga0.5As quantum wells exhibit the band edge at 0.78 eV.
There-5.2 Design concept to overcome Q-switch induced damage on SESAMs
900 950 1000 1050 1100 1150 1200
0 20 40 60 80 100
Reflectivity (%)
Wavelength (nm) 22°C
30°C 40°C 50°C 63°C 75°C 85°C 100°C
Figure 5.9: Static reflectivity of heated SAM-T2, measured with the FTIR-spectrometer. The Yb:YAG laser wavelength of 1030 nm is marked with a black line and the reflectivity values at 1030 nm are used as linear reflectivity values in further calculations and fit routines.
fore, the excess energy of photons within the energy range around 1.2 eV is lower for SAM-T2 than for SAM-T1, resulting in a lower absorption for SAM-T2 than SAM-T1 and consequently weaker temperature dependent changes of the absorption spectra.
An additional explanation for the difference in the temperature dependence of the static reflectivity spectra of the two SESAMs might be the different amount of quan-tum wells. For the low modulation depth of SAM-T2 just a couple of quanquan-tum well absorbers, up to a maximum of three, are required. For a modulation depth of 20 % a larger amount of quantum absorbers might be required. Hence, temperature de-pendent absorption changes within one single quantum well sum up more strongly in SAM-T1 than in SAM-T2. Furthermore, SAM-T1 shows the behavior of a SESAM with resonant coating, whereas SAM-T2 has an antiresonant top mirror.
As apparent in Fig. 5.9 the reflectivity of the stopband in the spectra of SAM-T2 is just around 80 %. Such a low linear reflectivity and the modulation depth of just a few percent results in a low nonsaturable reflectivity of < 90 % corresponding to nonsaturable losses of more than 10 %. However, these nonsaturable losses seem to be too high. As it is described in Section 4.1, a reference measurement is necessary to get an absolute reflectivity measured with the FTIR-spectrometer. From the very low stopband reflectivity I conclude that the sample used for reference measurements was
22◦C 30◦C 40◦C 50◦C 63◦C 75◦C 85◦C 100◦C
Fsat (µJ/cm2) 95 84 83 78 68 83 67 77
Rlin/Rns (%) 95.4 95.4 95.4 95.2 95.1 94.2 94.7 93.8
∆Reff/Rns (%) 3.7 3.8 3.9 4.1 4.3 4.8 4.7 5.1
Rlin, FTIR (%) 83.1 82.6 82.2 81.8 81.5 81.0 80.6 79.9
Rns (%) 87 87 86 86 86 86 85 85
∆Reff (%) 3.2 3.3 3.4 3.5 3.7 4.1 4.0 4.3
τslow (ps) 20 20 20 20 20 20 20 20
Table 5.2: Characterizing parameters for heated SAM-T2 extracted from pump-probe measurements and static FTIR-spectra. The time constants are roughly extracted from the transients at a fluence of 50 µJ/cm2, which are shown in Fig. 5.12.
100 101 102 103
79 80 81 82 83 84 85 86 87
Fluence (µJ/cm2)
Reflectivity (%)
22°C 30°C 40°C 50°C 63°C 75°C 85°C 100°C
Figure 5.10: Nonlinear reflectivity curve of heated SAM-T2, experimental results (dots) and fit curve (line). Absolute values based on PPS data and linear reflectivity values extracted from FTIR-spectra.
bad. Nevertheless, we took these static reflectivity measurements for further analysis.
For example we also extracted the linear reflectivity of SAM-T2 from the FTIR-spectra as we have already done it for SAM-T1. The reflectivity values at 1030 nm are used as linear reflectivity in further calculations and fit routines.
We also characterized SAM-T2 in the pump-probe setup for different temperatures.
All characteristic parameters are listed in Table 5.2. The temperature dependence of the nonlinear reflectivity curves of SAM-T2 is comparable to the behavior of SAM-T1.
5.2 Design concept to overcome Q-switch induced damage on SESAMs
Figure 5.11: Selected characterizing parameters for heated SAM-T2, temperature dependent. Left: Comparison of the fit parameter Rlin/Rns extracted from pump-probe measurements with the static, linear reflectivity values Rlin,FTIR measured with the FTIR-spectrometer. Based on these two parameters the nonsaturable reflectivity Rns was calculated. Right: Linear reflectivity and effective modulation depth versus SESAM temperature.
As it can be seen in Fig. 5.10 and in the right part of Fig. 5.11 the linear reflectivity decreases by ca. 3 % whereas the effective modulation depth increases by ca. 1 %. As already mentioned in the previous section the behavior of Rlin and ∆Reff is based on the fact that in this temperature range the nonlinear losses of SESAMs should nearly be constant as it is for SAM-T1. Even the nonsaturable losses of SAM-T2 increase by just a few percent with increasing temperature. Thus, it can be assumed that for temperatures <100◦C the nonsaturable losses are temperature independent. The temperature dependence of the nonlinear reflectivity curves of SAM-T1 and SAM-T2 seems to be a common SESAM behavior as another temperature dependent SESAM characterization shows the same effects [Sar12c].
As for SAM-T1 the carrier dynamics of SAM-T2 do not change due to the heating as it can be seen in the normalized transients in Fig. 5.12. To ensure that nonlinear effects are avoided the time constants are extracted from transients at fluence values below the saturation fluence. Due to a lower signal the transients of SAM-T1 are noisier compared to those of SAM-T2. Furthermore, due to a lower signal the normalized transients of SAM-T2 at low temperatures show more noise than those of high tem-peratures. Nevertheless, it is obvious that the time constants of the exponential decay are 20-25 ps and do not show a temperature dependence.
In conclusion, SAM-T1 with a high effective modulation depth and SAM-T2 with a low effective modulation depth show the same effects when heated: With raising tem-perature the linear reflectivity decreases whereas the nonsaturable losses remain nearly constant resulting in increased modulation depths. Time resolved reflectivity transients exhibit constant decay times independent of the SESAM temperature indicating that the carrier dynamics within the absorbers are not affected by the temperature increase.
0 10 20 30 40 50 60 70 80
−0.2 0 0.2 0.4 0.6 0.8 1
1/e
∆ R/R lin (norm.)
Time delay (ps)
22°C 30°C 40°C 50°C 63°C 75°C 85°C 100°C
Figure 5.12: Normalized reflectivity transients of SAM-T2 at a fluence of 50µJ/cm2 for different SESAM temperatures. The horizontal black line marks the reflectivity decrease of 1/e.
These temperature dependent effects of SESAMs can be used in a resonator. By con-trolling the SESAM’s temperature its modulation depth can be adjusted in-situ for best mode-locking conditions of the laser. This approach is already realized by Klopp et al. who increased the modulation depth of a SESAM by increasing the SESAM temperature [Klo11].
According to Eq. (2.15) Q-switching instabilities can be suppressed by a small modu-lation depth. To eliminate Q-switching instabilities at the beginning of the power-up phase the modulation depth should be small and with increasing power the modu-lation depth should increase, too. These changes of the modumodu-lation depth might be realized with a temperature regulated SESAM, having a low temperature, respectively low modulation depth, for low power and for raising power an increased modulation depth generated by a higher SESAM temperature.
Furthermore, these measurements proofed that the behavior of the SESAM strongly depends on its temperature and underpin the design concept presented in Section 5.2.1.
By an appropriate choice of the quantum well material and the temperature range of the SESAM during the power-up phase the band gap energy can be changed in this way that the Q-switching regime is suppressed. Only for high intracavity power, re-spectively high SESAM temperatures, the quantum wells absorb the laser light leading to cw mode-locking.